Various herbicides in free acid form, such as many auxin herbicides, have at least one carboxylic acid moiety. Auxin herbicides having at least one carboxylic acid moiety include 3,6-dichloro-2-methoxybenzoic acid (dicamba); 2,4-dichlorophenoxyacetic acid (2,4-D); 4-(2,4 dichlorophenoxy)butanoic acid (2,4-DB); 2-(2,4-dichlorophenoxy)propanoic acid (dichloroprop); 2-(4-chloro-2-methylphenoxy)acetic acid (MCPA); 4-(4-chloro-2-methylphenoxy)butanoic acid (MCPB); 4-amino-3,6-dichloro-2-pyridinecarboxylic acid (aminopyralid); 3,6-dichloro-2-pyridinecarboxylic acid (clopyralid); 2-[(4-amino-3,5-dichloro-6-fluoro-2-pyridinyl)oxy]acetic acid (fluroxypyr); [(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid (triclopyr); 2-(4-chloro-2-methylphenoxy)propanoic acid (mecoprop); 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid (picloram); 3,7-dichloro-8-quinolinecarboxylic acid (quinclorac); and 6-amino-5-chloro-2-cyclopropyl-4-pyrimidinecarboxylic acid (aminocyclopyrachlor). In particular, dicamba and 2,4-D have proven to be effective auxin herbicides.
Generally, synthetic auxin herbicides such as dicamba and 2,4-D mimic or act like natural auxin plant growth regulators. Auxin herbicides appear to affect cell wall plasticity and nucleic acid metabolism, which can lead to uncontrolled cell division and growth. The injury symptoms caused by auxin herbicides include epinastic bending and twisting of stems and petioles, leaf cupping and curling, and abnormal leaf shape and venation.
In order to provide enhanced solubility and increased herbicide loading for greater ease in formulation, conventional dicamba and other carboxylic acid auxin herbicide products are typically formulated as water-soluble salt solutions. For example, CLARITY (available from BASF) is a formulation containing the diglycolamine salt of dicamba and BANVEL (available from Arysta LifeScience North America LLC) is a formulation containing the dimethylamine salt of dicamba.
Off-site movement is sometimes associated with auxin herbicides. Volatile auxin herbicides like dicamba and 2,4-D can, under certain conditions of application, volatilize into the surrounding atmosphere and migrate from the application site to adjacent crop plants, such as soybeans and cotton, where contact damage to sensitive plants can occur. Typical symptoms of injury to crop plants include leaf cupping, leaf malformation, leaf necrosis, terminal bud kill and/or delayed maturity.
Some strategies to reduce herbicide volatilization have focused on encapsulation. In one approach, dicamba is absorbed into solid phase natural or synthetic polymers. However, the resulting particle sizes are typically not suitable for spray application and therefore limited to granular drop application. Micro-encapsulation in a polymer shell is also known in the art, but the relatively high solubility of dicamba and its salts precludes successful use of the technology in aqueous suspensions, and commercial dicamba micro-encapsulation products have not been developed.
Thus, a need persists for the identification of alternative salts of carboxylic acid herbicides that are suitable for formulation into herbicidal application mixtures and/or concentrate compositions that exhibit acceptable stability and compatibility characteristics. Preferably, the carboxylic acid herbicide salts exhibit low volatility in herbicide compositions that are efficacious, yet non-phytotoxic to sensitive crops located in areas adjacent to the target site and resist adverse effects to their volatility characteristics when tank mixed with other co-herbicides such as glyphosate.